Invasive Shothole Borers and Fusarium Dieback: A New Marin Threat

A beetle smaller than a sesame seed may not look like much of a threat to a mature tree.

But invasive shothole borers (ISHB) don’t work alone. These tiny beetles tunnel into trees and introduce Fusarium fungi, which can interfere with the movement of water and nutrients and lead to a disease known as Fusarium dieback.

The beetles have already caused extensive tree damage in Southern California, and recent detections farther north are giving Bay Area communities new reason to pay attention.

For Marin homeowners, that doesn’t mean it’s time to panic. But it does mean it’s worth knowing what invasive shothole borers look like, which trees may be vulnerable, and what signs could indicate a problem. 

We’ll walk you through what you need to know about this emerging threat and how you can help protect the trees on your property.

Understanding Invasive Shothole Borers and Fusarium Dieback

Invasive shothole borers are tiny, non-native ambrosia beetles with an unusual relationship with fungi. The beetles tunnel into a tree’s wood, carrying fungi into the galleries they create and cultivating it as a food source. 

Unfortunately, that partnership can be devastating for the tree.

Meet the Beetles Behind the Damage

Several invasive beetles in the genus Euwallacea have now been detected in California, including the polyphagous, Kuroshio, and greater shothole borers. These tiny beetles measure only about 1/16 inch (roughly 2 millimeters) long.

Many ambrosia beetles primarily colonize trees that are already dead, injured, or stressed. Invasive shothole borers are particularly concerning because some species can attack and kill healthy trees. 

Female beetles tunnel into the trunks and branches of susceptible trees, creating galleries where they lay their eggs. They also introduce fungi into these galleries, essentially creating fungal gardens that feed developing beetles and their offspring.

Once mature, new female beetles leave the tree carrying fungi with them, allowing the beetle-fungus partnership to spread to additional trees.

How Shothole Borers Cause Fusarium Dieback

Tunneling causes physical damage, and the fungi the beetles introduce create an additional threat. 

As the fungi colonize the galleries and surrounding wood, they interfere with the tree’s ability to transport water and nutrients. Over time, affected branches may wilt and die back, and heavily infested trees can eventually die.

This combination of beetle infestation and fungal disease is known as the invasive shothole borer–Fusarium dieback complex.

A Growing Northern California Threat

For years, invasive shothole borers were primarily a Southern California problem, where the beetle-disease complex has killed tens of thousands of urban and riparian trees.

Trees Vulnerable to Invasive Shothole Borers

One reason invasive shothole borers are such a concern is their unusually broad host range. They can attack hundreds of plant species, and dozens of trees and shrubs can support both the fungi and reproducing beetle populations.

Several trees commonly found in Marin County are confirmed reproductive hosts, including: 

• California sycamore (Platanus racemosa): This native tree is among the species that can suffer severe dieback and death from the beetle-fungus complex.
• Bigleaf maple (Acer macrophyllum): Another California native, bigleaf maple can support reproducing beetles and may be killed by an infestation.
• Valley oak (Quercus lobata): These native oaks are susceptible to serious damage and are among the confirmed hosts that may be killed.
• Willows a: Several native species, including arroyo willow (Salix lasiolepis) and red willow (Salix laevigata) can suffer significant damage.
• Cottonwoods: Close relatives of willows, Fremont (Populus fremontii) and black cottonwoods (Populus trichocarpa) are also vulnerable to these beetles. 
• Coast live oak (Quercus agrifolia): One of Marin’s signature native trees, coast live oak is a confirmed reproductive host, although infestations do not typically kill the tree.

Other common landscape plants, including Japanese maple (Acer palmatum), English oak (Quercus robur), London plane (Platanus × acerifolia), avocado (Persea americana), acacia (Acacia spp.), and California buckeye (Aesculus californica), can also serve as reproductive hosts. 

Because the host range is so extensive, tree species alone can’t tell you whether invasive shothole borers are present. Knowing the signs of an infestation is equally important.

What Homeowners Actually See

You’re unlikely to spot an invasive shothole borer crawling across your tree. The beetles are tiny and spend most of their lives hidden inside the wood.

Instead, look for the evidence they leave behind. The clearest sign is a small, perfectly round hole in the trunk or branches, typically about the size of the tip of a ballpoint pen.

Depending on the tree species, the response to an invasive shothole borer attack can look quite different. Some trees ooze sap or gum, while others develop staining, boring dust, or signs of canopy decline. Watch for symptoms such as: 

• Staining around entry holes: The bark may develop dark, wet-looking or lighter, dry discoloration around areas where beetles have tunneled into the tree.
• Gumming or other exudate: Some trees respond to attack by producing thick gum or sap-like material around the holes.
• Fine boring dust or frass: Beetle activity may leave sawdust-like material on the bark or distinctive little projections of compacted frass.
• White, sugary buildup: Some hosts, particularly avocado trees, may develop a powdery white buildup sometimes described as a “sugar volcano.”
• Wilting and branch dieback: As Fusarium dieback progresses, leaves may wilt and branches may begin to die. Advanced infestations can lead to broader canopy decline.

Just note that not every hole, stain, or dying branch means invasive shothole borers are responsible. Other insects, diseases, and even physical injuries can produce similar symptoms, making professional tree and plant healthcare important when battling tree pests.

Protecting Marin Trees From Invasive Shothole Borers

With an emerging pest threat, early detection matters. Fortunately, there are practical steps you can take to support your trees and help prevent invasive shothole borers from spreading. 

For healthy, uninfested trees, focus on good tree care, early detection, and careful handling of potentially infested wood: 

• Keep trees healthy: Proper watering, good soil conditions, a 2- to 4-inch-thick layer of organic mulch, and protection from mechanical damage can help trees remain healthy and resilient.
• Monitor susceptible trees: Periodically check trunks and branches for small round holes, staining, gumming, frass, or unusual dieback. If something changes, catching it early gives you more management options.
• Don’t move potentially infested wood: Shothole borers can survive in cut wood for weeks or months. Buy firewood locally, and don’t transport wood from a suspected infestation to another property.
• Handle infested material carefully: Pruning or removing an infested tree doesn’t eliminate the problem if live beetles remain in the wood. Infested material may need to be chipped and properly composted, solarized, kiln-dried, or otherwise treated.
• Get suspicious symptoms evaluated: An ISA-certified arborist can assess the tree, determine whether treatment or pruning may be appropriate, and identify severely infested trees that may need to be removed.

For trees with active infestations, management depends on the tree species, severity of the infestation, condition of the tree, and potential risk to people or property. 

Some lightly or moderately infested trees may benefit from professional treatment or removal of affected branches, while severely infested trees may be unlikely to recover.

The important thing is to confirm the problem before treating a tree. Professional diagnosis can help you choose an appropriate response while avoiding unnecessary pesticide use. 

Invasive Shothole Borers in Marin: FAQ

As invasive shothole borers move farther north in California, Marin homeowners understandably have questions about what the threat means for their trees. Here are answers to some of the most common.

Are invasive shothole borers in Marin County?

Invasive shothole borers have not yet been confirmed as established in Marin County. However, recent detections in Northern California have brought the threat closer to the Bay Area, and Marin County is actively surveying for the beetles.

What is Fusarium dieback?

Fusarium dieback is the disease associated with the fungi invasive shothole borers introduce into trees. As the fungi colonize the wood, they can interfere with water and nutrient transport, causing branch dieback and, in susceptible trees, eventual death.

Can invasive shothole borers kill healthy trees?

Yes. Invasive shothole borers can attack apparently healthy trees as well as stressed ones. However, the outcome varies considerably by tree species, and not every tree that can support reproducing beetles is typically killed by an infestation.

How can I tell a shothole borer hole from other insect damage?

Invasive shothole borers holes are very small, perfectly round, and roughly the size of a ballpoint pen tip. Because other insects and injuries can leave similar marks, look for additional signs such as staining, gumming, frass, or branch dieback rather than relying on the hole alone.

Can a tree with invasive shothole borers be saved?

Sometimes. Management depends on the tree species, its overall condition, and how extensive the infestation has become. Early detection may provide more options, while severely infested or declining trees may need to be removed.

Should I treat my trees preventively?

No. Current University of California guidance does not recommend preventive pesticide treatment for healthy trees that aren’t infested. Monitoring susceptible trees and maintaining good tree health are more appropriate when the beetles haven’t been detected.

What should I do if I suspect invasive shothole borers?

Avoid cutting or moving potentially infested wood until you know what you’re dealing with. Have the tree evaluated by an ISA-certified arborist who can identify the problem and recommend appropriate next steps.

Protect Your Marin Trees With Treemasters

Invasive shothole borers may be an emerging threat, but they’re only one of the pests, diseases, and environmental stresses Marin trees face. Regular monitoring and proactive care can help you spot changes early and give your trees the support they need to stay healthy.

The ISA-certified arborists at Treemasters Tree and Garden Care have been caring for Marin County trees since 1987. 

Our team can evaluate suspicious symptoms, identify pest and disease problems, and recommend appropriate treatment or supportive care based on the needs of your trees and property.

If you’re concerned about unusual holes, staining, dieback, or other changes in your trees, don’t wait and wonder. Contact Treemasters Tree and Garden Care today to schedule an evaluation and get expert guidance on your next steps.

Sources and References

• Marin County – “Agriculture, Weights and Measures: FY 2025–26 Workplan”
  Marin County planning document identifying a countywide survey for invasive shothole borers as a current pest-detection priority.
  https://assets.marincounty.gov/marincounty-prod/public/2025-02/fy2025-26-february-budget-workshop-materials-vf-reduced-pdf.pdf
• Santa Clara County Division of Agriculture – “Invasive Shothole Borers (ISHB) FAQs”
  Bay Area resource covering invasive shothole borer biology, Fusarium dieback, signs of infestation, host trees, and the importance of early detection and monitoring.
  https://ag.santaclaracounty.gov/invasive-shothole-borers-ishb-faqs
• UC Statewide Integrated Pest Management Program – “Invasive Shothole Borers”
  Peer-reviewed Pest Notes covering ISHB identification and biology, reproductive hosts, Fusarium dieback symptoms, monitoring, tree care, wood disposal, and management options.
  https://ipm.ucanr.edu/home-and-landscape/invasive-shothole-borers/
• UC Agriculture and Natural Resources – “Invasive Beetle Known for Killing Trees Spotted in the Bay Area”
  Regional resource detailing the arrival of polyphagous shothole borer in San Jose, its beetle-fungus relationship, symptoms, and implications for Bay Area trees.
  https://ucanr.edu/node/145939/printable/print